You should be able to run anything that powers from the PCI x16 slot, including the newer 7xxx series. Where did you find limited capability for GPUs?
Here is a post on Tom's talking about running a 56xx card:http://www.tomshardware.com/forum/319153-33-upgrade-dell-inspiron-budget
Yeah, I would cancel that order and get yourself something that is much better.
BTW, it isn't Dell's fault. I have numerous Dells at home, one that is running SLI GTX260s w/ stock power supply. It is the decision to purchase that computer, that was designed to run what was purchased with it, that you should be questioning. Not the manufacturer.